Vibrating micro-electro-mechanical-system (MEMS) gyroscopes are used in a variety of mechanical or electro-mechanical systems where an angular rotation rate is to be measured. A vibrating MEMS gyroscope comprises a gyroscope mass that is connected by springs to a substrate. The gyroscope mass is movable along a driving axis in resonant oscillation by the use of a drive force in order to provoke and maintain the movement. The drive force is supplied and controlled using a drive actuation unit and a drive measurement unit and associated circuitry. The drive actuation unit typically comprises a capacitive coupling along the driving axis between a capacitor plate on the substrate and an opposite capacitor plate on the movable gyroscope mass. A Coriolis force acting on the gyroscope mass may be induced as a capacitive force by applying a voltage to the capacitor plates of the drive actuation unit, whereby the gyroscope mass is moved. The drive measurement unit comprises, e.g., a similar pair of capacitor plates. The capacitance between the capacitor plates of the drive measurement unit is measured as a drive measurement signal and forms an indication of the displacement of the gyroscope mass along the driving axis.
A Coriolis force will apply to the gyroscope proof mass in the presence of an angular rotation. The Coriolis force Fc is proportional to the product of the proof-mass ‘m’, the input rate ‘Ω’, the proof mass velocity ‘v’, and its angular rate of rotation perpendicular to the direction of movement. The Coriolis force hereby results in a displacement of the gyroscope mass along a sensing axis perpendicular to the driving axis. Measurement of the displacement of the gyroscope mass along the sensing axis can be used to obtain a measure of the Coriolis force and thus a measure of the angular rate of rotation.
A sense measurement unit is sometimes provided, which, similar to the drive measurement unit, may comprise a capacitive coupling along the sensing axis between a sense capacitor plate on the substrate and an opposite sense capacitor plate on the movable gyroscope mass. The capacitance between the sense capacitor plates of the sense measurement unit is measured as a sense measurement signal and forms an indication of the displacement of the gyroscope mass along the sensing axis.
Thus, the basic architecture of a vibratory gyroscope is comprised of a drive-mode oscillator that generates and maintains a constant linear momentum of the proof-mass, and a sense mode circuit that measures the sinusoidal Coriolis force induced due to the combination of the drive oscillation and any angular rate input. Since the Coriolis Effect is based on conservation of momentum, the drive-mode oscillator circuit is implemented to provoke the oscillation of the proof-mass that is the source of this momentum.
FIG. 1 illustrates a simplified block diagram of such a drive-mode oscillator circuit 100 for a MEMS, which uses a simple analog control loop to control the MEMS proof-mass displacement. The analog loop applies an electrostatic force proportional to displacement. The drive-mode oscillator circuit 100 comprises a capacitance to voltage (C2V) converter 110 arranged to convert a capacitance change of a MEMS drive measurement unit (DMU) (not shown) caused by the displacement of the proof-mass to a voltage measurement signal. An integrator 120 receives the voltage measurement signal and phase shifts it by, 90° in order to compensate for the phase lag (mechanical pole) of the MEMS device. A voltage gain amplifier (VGA) 130 receives the phase shifted voltage signal and outputs an actuation voltage signal to a drive actuation unit (DAU) (not shown) of the MEMS device. An automatic gain control (AGC) circuit 140 provides a control signal to the VGA 130 in order to control the amplitude of the actuation voltage signal output thereby.
FIG. 2 illustrates a drive activation waveform 200 associated with FIG. 1. At start-up, since there is no displacement, the electrostatic force is null. Only the force generated by the noise may make the loop start. A system turn-on time is indicated at 210 and FIG. 2 illustrates the drive start-up time 265 that is required to reach the target MEMS displacement level. Due to the positive feedback of the drive loop in FIG. 1, the drive motion is amplified 230 until the target displacement is reached at 220. Thus, when a MEMS gyroscope is initially turned on, it takes a significant amount of time to achieve an oscillating displacement range for the proof-mass in order to obtain meaningful measurements. Thereafter, the AGC loop is arranged to reduce the drive actuation signal 235 in order to regulate the capacitance to voltage (C2V) level 250.
The start-up time for gyroscopes is a known weakness due to the need for the proof-mass motion to reach its natural (resonance) frequency from an inertia state. Furthermore, the natural (resonance) frequency of such a proof-mass is not known apriori, or indeed is it consistent between, say, gyroscopes, due to, inter alia, manufacturing and component tolerances, open-loop operation and interaction between, say, an electrical resistor-capacitor (RC) oscillator and the mechanical resonator. The start-up time for gyroscopes is also a known weakness due to the fact that the bandwidth of the mechanical resonator has to be small, and is therefore difficult to tune. In order to avoid start-up delays in the use of such MEMS gyroscopes, it is known to implement a ‘standby mode’ in which oscillation of the proof-mass is maintained by the drive-mode oscillator circuit, whilst the sensing circuit(s) is/are powered down to conserve power. However, maintaining such a standby mode of the drive-mode oscillator circuit requires the oscillator circuit to remain powered up. Furthermore, driving the proof-mass consumes a significant amount of power, which in many electronic applications is undesirable.
A further drawback of such a MEMS device is the high quality (‘Q’) factor (e.g. a ‘Q’ value of ˜10,000) of the MEMS proof mass oscillation system. Such a ‘Q’ factor inherently provides a slow gain increase and therefore requires a long time in order to reach the correct amplitude of oscillation.
U.S. Pat. No. 7,895,893 B2, titled: “Method for operating a vibrating gyroscope and sensor”, describes a vibrating gyroscope that uses a fixed frequency kick of a value of a natural frequency based on a previously measured value, in order to speed up the start-up time.